Power semiconductor devices are widely used to carry large currents and support high voltages. Modern power devices are generally fabricated from monocrystalline silicon semiconductor material. One widely used power device is the power Metal Oxide Semiconductor Field Effect Transistor (MOSFET). In a power MOSFET, a control signal is supplied to a gate electrode that is separated from the semiconductor surface by an intervening insulator, which may be, but is not limited to, silicon dioxide. Current conduction occurs via transport of majority carriers, without the presence of minority carrier injection that is used in bipolar transistor operation. Power MOSFETs can provide an excellent safe operating area, and can be paralleled in a unit cell structure.
As is well known to those having skill in the art, power MOSFETs may include a lateral structure or a vertical structure. In a lateral structure, the drain, gate and source terminals are on the same surface of a substrate. In contrast, in a vertical structure, the source and drain are on opposite surfaces of the substrate.
One widely used silicon power MOSFET is the double diffused MOSFET (DMOSFET) which is fabricated using a double-diffusion process. In these devices, a p-base region and an n+ source region are diffused through a common opening in a mask. The p-base region is driven in deeper than the n+ source. The difference in the lateral diffusion between the p-base and n+ source regions forms a surface channel region.
Recent development efforts in power devices have also included investigation of the use of silicon carbide (SiC) devices for power devices. Silicon carbide (SiC) has a combination of electrical and physical properties that make it attractive for a semiconductor material for high temperature, high voltage, high frequency and high power electronic devices. These properties include a 3.0 eV bandgap, a 4 MV/cm electric field breakdown, a 4.9 W/cm-K thermal conductivity, and a 2.0×107 cm/s electron drift velocity.
Consequently, these properties may allow silicon carbide power devices to operate at higher temperatures, higher power levels and/or with lower specific on-resistance than conventional silicon-based power devices. A theoretical analysis of the superiority of silicon carbide devices over silicon devices is found in a publication by Bhatnagar et al. entitled “Comparison of 6H—SiC, 3C—SiC and Si for Power Devices”, IEEE Transactions on Electron Devices, Vol. 40, 1993, pp. 645-655. A power MOSFET fabricated in silicon carbide is described in U.S. Pat. No. 5,506,421 to Palmour entitled “Power MOSFET in Silicon Carbide” and assigned to the assignee of the present invention.
4H—SiC Power DMOSFETs have the potential to offer significant advantages over conventional high voltage Si power switches. Unfortunately, however, it may be difficult to thermally grow an acceptable gate oxide for these devices. Much effort has been focused on reducing the interface trap density (DIT) at the SiC/SiO2 interface in order to increase the channel mobility (μCH) of the devices. Nitric Oxide (NO) anneals at 1175° C. have increased the μCH from single digits to ˜30 cm2/Vs. See, e.g., G. Y. Chung, et al., IEEE Electron Dev. Let. 22, 176 (2001). Researchers have demonstrated even higher channel mobility (˜150 cm2/Vs) by oxidizing in an environment containing metallic impurities. See, e.g., U.S. Pat. No. 6,559,068 . However, such a process may result in significant oxide contamination, may provide an uncontrolled oxidation rate (tOX>1500 Å), and/or may be incompatible with high temperature processing steps such as may be used for ohmic contact anneals.